    Patent Document 1: JP-A S61-36907    Patent Document 2: JP-A 2005-123454    Patent Document 3: JP-A 2005-48213    Patent Document 4: JP-A 2005-97055    Patent Document 5: JP-A 2007-165782
Some metals and metal compounds such as metal oxides, metal nitrides or metal sulfides have so-called magnetic property such as being attracted or repulsed to a magnet. Those having such property are referred to as magnetic bodies or magnetic substances. Microparticles of the magnetic body, that is, magnetic microparticles are utilized in a high density recording medium or magnetooptical recording medium used for such as computers, a magnetic tape, a electromagnetic wave-shielding material, a toner for electrophotograph and the like due to their magnetic properties. Further, a water purification agent such as an aggregating agent comprising a magnet-binding polymer has been developed and a drug delivery system using a magnetic imparting agent has also been developed in pharmaceutical industries.
Magnetic fluids are those in which the magnetic microparticles described above are dispersed in a medium such as oils or water in a colloidal state. At the beginning of the 1960's, in NASA's space program, magnetic fluids were developed for utilizing as the magnetic transporting control of liquid fuels for rockets under weightless conditions in space, or as the liquid encapsulation materials for space suits. Such magnetic fluids are used at many sites such as an angle sensor, an inclination sensor, a shaft seal and a vibration damper, and a research for improving the property of the magnetic fluids has been promoted. For example, in a research for enlarging the saturation magnetization of the magnetic fluids, there has been disclosed a technique relating to magnetic fluids comprised of cobalt metal microparticles, surfactants and carbonaceous catalysts (Patent Document 1).
In order to utilize the property of the magnetic microparticles, it is very important to allow the magnetic microparticles to be monodispersed for maintaining a coercive force while having a high coercive force, that is, to prepare magnetic microparticles having a uniform size, and there is also the need for controlling the crystallinity and crystal form thereof.
The production methods include a method for obtaining magnetic fluids containing metal microparticles having an average particle size of 7 nm to 12 nm in high concentration by adding metal carbonyl to a hydrocarbon medium in which a surfactant is dissolved and heating the mixture to be thermally decomposed as described in Patent Document 1, a method for controlling the particle size of metal microparticles to be prepared by the reflux temperature of an organometallic compound solution dispersed in a solution when metallic magnetic fluids are produced in a solution in the presence of a surfactant as described in Patent Document 2, and a method for irradiating a dispersion slurry comprising amorphous particles having a particle size of 0.5 nm to 100 nm whose surface is bound to organic coordination molecules with laser to heat the amorphous particles, followed by crystallization (Patent Document 3). Further, a method for producing composite oxide microparticles comprising the calcining step of mixing and calcining two kinds of metallic oxide microparticles, at least one of which is a ferromagnetic oxide, in a vapor medium at high temperature to prepare composite oxide microparticles in which both microparticles are melt and joined, and the rapid cooling step of subjecting the vapor medium passed through the calcining step to a magnetic field to isolate the composite oxide microparticles from the vapor medium and simultaneously to introduce the composite oxide microparticles into the cooling system, followed by preparation of amorphous composite oxide microparticles by rapid cooling (Patent Document 4), and a method for preparing magnetic particles or precursors of magnetic particles by introducing a raw material gas flow containing a metal constituting a magnetic body and a reactant gas flow covering the raw material gas flow into a reaction space in a high-temperature atmosphere, forming microparticles through heat reaction at the periphery of the raw material gas flow and simultaneously cooling the microparticles with the reactant gas flow (Patent Document 5).
However, the production method in which two or more kinds of organic solvents are necessarily used in a large amount as described in Patent Document 2, or the production method in which magnetic microparticles after reaction are subjected to specific post treatment or process as described in Patent Document 3 highly possibly allows the steps to be complicated, and the time requiring for the production to be prolonged. When the method such as Patent Document 4 or 5 is used, there is a problem such that an apparatus having an expensive and complicated mechanism is necessarily used and therefore, it is concerned that the value of a product is naturally high. All the problems lie in the fact that a problem of aggregation and control of crystallinity, which are easily caused upon obtaining nano size magnetic microparticles, remain unsolved. That is, in the liquid phase reaction capable of easily producing magnetic microparticles, all the problems lie in the fact that homogenization of particle size and unification of a reaction product can not be effectively and efficiently carried out due to temperature nonuniformity and stirring nonuniformity in a reactor.
In light of this situation, it is an object of the present invention to provide a method for producing magnetic microparticles, the magnetic particles are obtained by allowing a magnetic raw material to react with a magnetic microparticles-separating agent in a thin film fluid formed between two processing surfaces arranged to be opposite to each other so as to be able to approach to and separate from each other, at least one of which rotates relative to the other, wherein the temperature in the thin film fluid is highly uniform and the stirring in a reactor is also highly uniform so that monodispersed magnetic microparticles can be prepared depending on its purpose, and wherein clogging with a product does not occur due to self-dischargeability, great pressure is not necessary, and productivity is high, magnetic microparticles obtained therefrom, and a magnetic fluid containing the same.